Electronegativity+and+Polarity

Electronegativity is a measure of the tendency of an atom to attract electrons in a chemical bond. The most highly electronegative elements occur in the upper right in the periodic table (disregarding the noble gases, which rarely form compounds). The least electronegative elements occur in the lower left of the table.

The electronegativity difference between two bonded atoms can be used to predict the character of the bond. A bond is nonpolar covalent when there is virtually no electronegativity difference. A bond is polar covalent when there is some electronegativity difference (unequal electron sharing). A bond is ionic if the electronegativity difference is very large. An electronegativity of more than 1.7 generally means that a bond is more ionic than covalent.

A molecule also may be polar or nonpolar, depending on whether it contains polar bonds and depending on its shape. for example, a water molecule is polar because there is a significant electronegativity difference between the H and O atoms and because the molecule is bent, not symmetric. There is a negative end near the more electonegative O atom, and a positive end near the H atoms. Carbon tetrachloride (CCl4) has polar bonds, but the tetrahedral molecule as a whole is nonpolar because it is symmetric.

The properties of covalent substances vary, depending on a number of factors, including the intermolecular forces. Polar molecules tend to be soluble in polar solvents, whereas nonpolar molecules tend to be soluble in nonpolar solvents. For nonpolar molecules, the intermolecular forces are weak dispersion forces, and melting and boiling points tend to be low. For polar molecules, there are stronger dipole-dipole forces whose magnitute depends on the degree of polarity. If hydrogen is bonded to a highly electronegative atom, such as oxygen, the intermolecular force can be quite strong and is called hydrogen bonding. Such covalent substances have relatively high boiling pints for their molecular size. In some covalent substances, the atoms are connected in large netwrok solids, are very hard and have very high melting points.